*Proverbial* means “referred to in a proverb or idiom”. The proverb referred to here is [“To a man with a hammer, everything looks like a nail”][1] (variants include “He that is good with a hammer tends to think everything is a nail”). The overall meaning is you should not get so kind of a given tool (programming language, or pattern) that you use it exclusively, regardless of whether it is fit for the job at hand.


  [1]: http://en.wikipedia.org/wiki/Law_of_the_instrument